In the finishing of metal products it is common to transfer the workpieces upon heat treatment thereof to a gag press for straightening. The workpieces are subsequently examined for internal structural flaws at a testing station remote from the pressing station. In addition to the identification, localization and evaluation of internal defects, dimension measurements are implemented at the testing station for checking whether the processed workpieces conform to predetermined tolerances.
The reliability and speed of the straightening and testing operations are significant in determining the cost and the quality of construction of machinery elements.
The flaw-search techniques, such as magnetic-powder testing, X-ray examination and ultrasonic-wave monitoring, have proceeded substantially out of the experimental stage and have reached a high level of development. There are also testing techniques which yield results in the form of output signals to be used in subsequent processing operations.
A particularly effective and versatile flaw-search technique which has been recently developed comprises the analysis of pressure waves or sound waves which are emitted by a workpiece upon the striking thereof. Descriptions of the state of this art are to be found in "Metal Surfaces" ("Metalloberflache"), No. 5, 1979, published by the Hanser-Verlag in Munich, West Germany, and in an intercompany communication from the Firm Fischer-Pierce-Walzburg of Leutkirch, West Germany, to the Zahnradfabrik Friedrichshafen, West Germany, via M. Bentley, Dunegan-Endevco (Congress 1978). The utilization of such a pressure-wave analysis technique at the testing station is, however, insufficient to enable the localization of defect causes. Because defects may be undetectably induced by the pressing apparatus itself, many workpieces sorted out at the testing station for further heat treatment and subsequent straightening will only be subjected to additional flaw-producing stressing in the press and thereby become unsuitable for the intended application. Such destruction of blanks in a refinishing process is a waste of time and energy.
Sampling the workpieces at the output of the gag press to save time results in a decrease in quality, and while it is possible to insert another testing station between the heat-treatment plant and the gag press, such an additional station would involve an increased expenditure of time and energy and a greater investment in conveyance devices and in space.
Another disadvantage of known applications of the above-mentioned pressure wave analysis technique is the difficulty of making valid comparisons of the test results of different blanks of the same production series or of the same blank before and after a second heat treatment and straightening operation, this difficulty being due at least in part to the impossibility of duplicating conditions at the testing station. Not only must each blank under comparison be supported at analogous positions and struck with the same energy at similar points, but the pressure-wave sensors of the examining apparatus must be brought to engage the blanks at similar positions on the surface thereof. Such a placement of the sensors requires highly skilled technicians.
If the testing station downstream of the press is particularly remote therefrom, the increased delay between bending and examining aggravates the difficulty of localizing sources of defects, thereby raising time and energy losses.